1. Field of the Invention
The present invention relates to a process for the preparation of praseodymium (Pr) metal, a praseodymium-neodymium (Pr-Nd) alloy, or an iron alloy thereof (Pr-Fe or Pr-Nd-Fe). More particularly, the present invention relates to a process for preparing the above-mentioned metal or alloy by the fused salt electrolytic method using praseodymium fluoride or praseodymium fluoride and neodymium fluoride as the starting material. Especially, the present invention relates to a process in which a high-purity metal or alloy suitable for a magnetic material of the Pr type or Pr-Nd type, which recently has attracted attention for high-performance magnets, can be manufactured at a low cost.
2. Description of the Related Art
As a relatively cheap high-performance permanent magnet, there has recently been proposed a magnet formed by substituting a part of the Sm type magnet composition by Pr or a Pr-Nd alloy, or a permanent alloy of the (Pr, Nd)-Fe-B type or (Pr, Nd)-Fe-Co-B type. As the method for preparing Pr, Pr-Nd, or an iron alloy thereof to be used for such permanent alloys, the following methods are known.
(1) A method comprising reducing a Pr compound or Pr-Nd compound with an active metal such as metallic calcium.
(2) A method comprising carrying out alloying reaction between Pr oxide or Pr-Nd oxide and iron used as the cathode by fused salt electrolysis to collect Pr, Pr-Nd, or an Fe alloy thereof (E. Morrice et al, Bur. Mine Rep. Invest., No. 7146, 1968).
(3) A method comprising carrying out an alloying reaction between Pr fluoride or Pr-Nd fluoride and iron used as the cathode by the fused salt electrolysis to collect a Pr-Fe or Pr-Nd-Fe alloy (Japanese Unexamined Patent Publication No. 61-253391).
These three methods are now compared with one another. The first method is disadvantageous from the economical viewpoint and in view of the low productivity because an expensive metal such as calcium is used as the reducing agent and the reaction is carried out batchwise.
In the second method, since the solubility of the oxide in the fused salt is low, if the oxide is fed in an amount exceeding the solubility, the oxide is incorporated in the lower portion of the fused salt, that is, in the deposited metal, to form a mixture of the metal with the fused salt and oxide, and the second method is defective in that recovery of a high-grade metal is difficult.
In the third method, the dissolution of the fluoride in LiF as the fused salt forms a eutectic mixture and the dissolution range is broad. The metal can be recovered without trouble as observed in the oxide electrolysis method. Accordingly, the third method is excellent in this point.
However, as the result of investigations made by the inventor, it has been confirmed that the method disclosed in Japanese Unexamined Patent Publication No. 61-253391 has problems described below and is not suitable for preparing a Pr-Fe or Pr-Nd-Fe alloy economically advantageously on an industrial scale.
According to the method disclosed in Japanese Unexamined Patent Publication No. 61-253391, Pr fluoride or a mixture of Pr fluoride and Nd fluoride is maintained at a concentration of 35 to 76% by weight in an electrolytic bath composed mainly of LiF and a Pr-Fe or Pr-Nd-Fe alloy is deposited while maintaining the temperature of the electrolytic bath at 770.degree. to 950.degree. C. The reason why the amount of PrF.sub.3 as the starting compound or the starting PrF.sub.3 -NdF.sub.3 mixture is limited to 35 to 76% by weight is as follows. The phase diagram of LiF-PrF.sub.3 shown in FIG. 5 is the eutectic phase diagram. The melting points of LiF, PrF.sub.3, and LiF-PrF.sub.3 at the eutectic point (the composition where the amount of PrF.sub.3 is about 67% by weight) are about 850.degree. C., higher than about 1300.degree. C. and about 730.degree. C., respectively. Accordingly, it is considered that the above-mentioned composition of the LiF-PrF.sub.3 system is selected so that the melting point of the bath is lower than about 840.degree. C.. The phase diagram of the LiF-NdF.sub.3 is substantially the same as described above, and the same can be said with respect to the bath composition for the production of a Pr-Nd-Fe alloy (R. E. Thoma, Progress in Science and Technology of the Rare Earths, Vol. 12, page 110, Pergamon Press, New York, 1966). In the production of a metal by the fused salt electrolytic method, for the reasons set forth below, it is preferred that the electrolysis temperature be lower. Accordingly, it is considered that the composition of the fused salt be such that the operation is carried out on the fused salt having a low melting point. It is considered that the above-mentioned composition is selected from this viewpoint.
(1) When the operation is carried out at a low temperature, the damage of the electrolytic cell material is small, and also the evaporation loss of the electrolytic bath is small.
(2) An energy-saving effect is attained when the operation is carried out at a lower temperature.
(3) In the fused salt electrolysis, the once deposited metal is formed into metal mists again and these metal mists react with the bath components to form the starting material. This reaction is conspicuous as the temperature is higher. It is considered that the ratio of the actually recovered amount of the metal to the theoretical value, calculated based on this balance, that is, the current efficiency, is reduced as the temperature is higher.
However, as the result of investigations made by us, it was confirmed that in the fluoride electrolysis, unlike the known oxide or chloride electrolysis, the reaction is advanced through a complicated mechanism, and the above-mentioned composition is not economically desirable.
The following two points are important for producing a metal economically advantageously by the fused salt electrolysis.